Added VerifyNothing() to assert that there is no any calls in mock

I've added VerifyNothing() to assert that there is no any calls in mock.

Currently if you need to assert if method was not called, you need to check using .Verify(o => o.Action(), Times.Never()) method. Sometimes it leads to errors. If you change production code and replace Action with another method (like TryAction or something else), the test will become green and might lead to the bugs.

In some cases it is possible to assert that nothing was called in the mock. I've added VerifyNothing() method for this purposes.